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Carbohydrate Polymers

journal homepage: www.elsevier.com/locate/carbpol

Review

Stimuli responsive ion gels based on polysaccharides and other polymersprepared using ionic liquids and deep eutectic solvents

Kamalesh Prasada,b,⁎, Dibyendu Mondalc, Mukesh Sharmad, Mara G. Freired,Chandrakant Mukeshe, Jitkumar Bhatta,b

a Natural Products and Green Chemistry Division, CSIR-Central Salt &Marine Chemicals Research Institute, G. B Marg, Bhavnagar 364002, Gujarat, Indiab AcSIR- Central Salt &Marine Chemicals Research Institute, G. B Marg, Bhavnagar 364002, Gujarat, Indiac Sustainable Energy Materials and Processes Group, Centre for Nano and Material Science, Jain University, Bangalore 562112, Indiad CICECO—Aveiro Institute of Materials, Chemistry Department, University of Aveiro, 3810-193 Aveiro, Portugale Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India

A R T I C L E I N F O

Keywords:Ion gelIonic liquidDeep eutectic solventPolysaccharidesBiopolymerCo-polymerStimuli responsive

A B S T R A C T

Ion gels and self-healing gels prepared using ionic liquids (ILs) and deep eutectic solvents (DESs) have beenlargely investigated in the past years due to their remarkable applications in different research areas. Herewithwe provide an overview on the ILs and DESs used for the preparation of ion gels, highlight the preparation andphysicochemical characteristics of stimuli responsive gel materials based on co-polymers and biopolymers, withspecial emphasis on polysaccharides and discuss their applications. Overall, this review summarizes the fun-damentals and advances in ion gels with switchable properties prepared using ILs or DESs, as well as theirpotential applications in electrochemistry, in sensing devices and as drug delivery vehicles.

1. Introduction

Stimuli-responsive polymeric materials, especially gel like mate-rials, are receiving considerable attention from the scientific commu-nity due to their potential applications for drug delivery, as biosensors,shape memory materials, coating and textiles (Stuart et al., 2010; Yangand Urban, 2013). Almost all polymeric gels, irrespective of their syn-thetic or bio-based origin, show both liquid like flow and elastic be-haviour. These characteristics make them good candidates for the fab-rication of functional materials. Self-repairing is one of the mostimportant functionalities of gelling materials, being investigated bymany researchers. Among the numerous approaches to introduce theself-repairing ability in gels, one of the most popular is based on theaddition of cross-linkers (Urban, 2012). There are several reports ofself-healing polymeric gel systems fabricated mainly by cross-linkingwith groups containing the eNHeCOe moiety capable of forming re-versible covalent bonds (Hager, Greil, Leyens, van derZwaag, & Schubert, 2010; Kushner, Vossler, Williams, & Guan, 2009;Phadke et al., 2012). The formation of these self-healing materials in-volves the sharing of electrons among the host and the guest moleculesleading to the formation of covalent bonds. However, in biologicalliving systems, reversible noncovalent molecular interactions play animportant role, for instance in the replication of DNA, in the folding of

proteins into intricate three-dimensional forms, and in the detection ofmolecular signals (Berg, Tymoczko, & Stryer, 2002).

The design of stimuli responsive-healing polymeric systems involvesthe encapsulation of effective healing agents in desired polymeric sys-tems followed by activation employing external stimuli, such asheating, which is a popular way to regain the structural integrity instructurally damaged polymers (Cho, Kim, Oh, & Chung, 2010). Afterheating, the healing agent moves to the damaged part and promotes theself-healing by initiating suitable polymeric entanglements. In suchphysically induced healing processes, heating promotes the movementof polymeric chains and diffusion. In such cases the polymeric materialis usually heated above its glass transition temperature (Tg) (Prageret al., 1981). The heating above Tg promotes surface rearrangement,followed by wetting, diffusion and re-entanglement of the polymerchains (Kim&Wool, 1983).

Smart polymeric materials are materials that are responsive to dif-ferent stimuli, such as pH, temperature, light, solvent, magnetic field,redox and mechanical stress. Such materials have potential interest forvarious fields of applications (Klaikherd, Nagamani, & Thayumanavan,2009; Balu et al., 2014; Zhuang, Gordon, Ventura, Li, & Thayumanavan,2013), such as in drug delivery (Li et al., 2014; Liu, Zhou, Guan,Su, & Dong, 2014; Lee, Lee, & Park, 2014), imaging (Sundaresan,Menon, Rahimi, Nguyen, &Wadajkar, 2014), catalysis (Deng et al.,

http://dx.doi.org/10.1016/j.carbpol.2017.10.020Received 16 August 2017; Received in revised form 3 October 2017; Accepted 3 October 2017

⁎ Corresponding author at: CSIR- Central Salt &Marine Chemicals Research Institute, G. B Marg, Bhavnagar 364002, Gujarat, India.E-mail addresses: kamlesh@csmcri.res.in, drkamaleshp@gmail.com, kamlesh@csmcri.org (K. Prasad).

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2014), coatings (Zhao et al., 2014), and sensors (Kavanagh, Byrne,Diamond, & Fraser, 2012; Zhang, Jin, Zheng, & Duan, 2014; Li, Xiao,Lin, &Wang, 2012; Yan et al., 2012). Several authors have reviewedstimuli responsive polymeric gels functionalized with metals (Weng,Fang, Zhang, Peng, & Lin, 2013), carbon nanotubes, graphene andmulti-responsive complexes (Tang, Kang, Wei, Guo, & Zhang, 2013).

In recent years, ionic liquids (ILs) emerged as unique platforms formaterials design (Noro, Matsushita, & Lodge, 2008; Smiglak et al., 2014)and as suitable solvent media for the dissolution of several poly-saccharides, including cellulose, starch, chitin and DNA (Swatloski, Spear,Holbrey, &Rogers, 2002; Isik, Gracia et al., 2014; Isik,Sardon, &Mecerreyes, 2014; Payal, Bejagam, Mondal, & Balasubramanian,2015; Wilpiszewska& Spychaj 2011; Prasad, Izawa, Kaneko, & Kadokawa,2009; Prasad, Murakami et al., 2009; Mondal, Sharma, Mukesh,Gupta, & Prasad, 2013; Sharma, Mondal, Mukesh, & Prasad, 2013a;Sharma, Mondal, Mukesh, & Prasad, 2013b). Among these, DNA drawsspecial attention due to its molecular recognition, biocompatibility, bio-degradability and mechanical flexibility features, making it suitable for thedesign of functional materials useful in molecular sensing in the form ofDNA nanomachines and intelligent drug delivery and programmablechemical syntheses (Bath &Turberfield, 2007). DNA based hybrid mate-rials and hydrogels were proposed for applications in targeted drug de-livery, tissue engineering bioanalysis, biomedicine and as stimuli-re-sponsive materials (Lee et al., 2012; Um et al., 2006; Peng et al., 2013).Further, due to the responsiveness of DNA towards pH and salt addition, itis also considered as a suitable precursor for designing bio-artificial mus-cles (Costa, Miguel, & Lindman, 2007; Besteman, Eijk, & van Lemay,2007). However, from the DNA based ion gel point of view, there is onlyone report on the preparation of gelatin and DNA based gel polymerelectrolytes by treating them with acetic acid and LiClO4, leading to theformation of ion gels with high ionic conductivity (Pawlicka et al., 2009).There are also limited reports on the preparation of ion gels by the com-bination of DNA and ionic liquids (ILs) or deep eutectic solvents (DESs).However, ILs are extensively used as suitable substrates for the preparationof other ion gels (Isik, Gracia et al., 2014; Isik, Sardon et al., 2014; Sekiet al., 2005; Ueki and Watanabe, 2008).

The preparation of thermo-responsive synthetic polymers, such aspolymethyl methacrylates and poly (N-isopropylacrylamide) in[C2mim][N(Tf2)] (Fig. 1) was first described by Ueki in 2014. Multistimuli responsive polymers with switchable wettability functionalizedwith imidazolium based ILs were synthesised and proposed by Döbbelinet al. in 2009. Apart from the preparation of stimuli-responsive mate-rials comprising polymers in ILs, polymerizable ILs (PILs) were alsoproposed as suitable substrates for the preparation of functional stimuli-responsive materials and ion gels suitable for gas separations (Cowan,Gin, & Noble, 2016; Kausar, 2017). Well-defined triblock copolymers,namely polystyrene-block-poly(methyl methacrylate)-block-poly-styrene (SMS), were used to prepare viscoelastic ion gel with [C2mim][NTf2]) (Fig. 1) with tetrahydrofuran, displaying high ionic con-ductivity (Imaizumi, Kokubo, &Watanabe, 2012).

Up to date the research on functional materials has mainly focusedon synthetic thermosetting, thermoplastic and elastomeric polymersbased on reversible Diels-Alder reactions. Very less attention hashowever been given to the preparation of “smart” materials based onrenewable/natural polymers, which are of particular significance in linewith the principles of ‘green chemistry’ and sustainability(Höfer & Bigorra, 2008).

An overview of the current smart ion gels materials prepared usingco-polymers, biopolymers or polysaccharides and ILs or DES, as well astheir potential applications, are described and discussed below. Thefollowing section is presented according to the response of the ion gelsto external stimuli, namely pH, heat, light and magnetism. The che-mical structure of different ILs, DESs, and polymers, copolymers,

biopolymers and polysaccharides investigated for the preparation ofsuch materials are shown in Figs. 1 and 2.

2. pH-responsive ion gels

pH or proton (H+) responsive ion gels show different behaviourupon changes in the pH of the system (Noro et al., 2008; Mukesh,Bhatt, & Prasad, 2014; Hashimoto, Fujii, Nishi, Sakai, & Shibayama,2016; Xie, Huang, & Taubert, 2014). Towards the development of suchion gels, Xie et al. (2014) have reported dye-IL based proton responsivetransparent, ion-conducting, and flexible ion gels, which exhibit re-versible colour change depending on the concentration of protons orhydroxide ions. Researchers first prepared the imidazolium based ILhaving methyl orange [C4mim][MO] as counter ion (Fig. 1A), followedby the addition of poly(methylmethacrylate) (PMMA) and an anotherIL, namely [C4mim][N(Tf)2] (Fig. 1A), in the presence of acetone.Uniformity and absence of defects in the ion gels were confirmed byscanning electron microscopy (SEM), FT-IR spectroscopy and thermogravimetric (TGA) studies. Moreover, no significant shifts in FT-IRbands after the IL incorporation were observed, indicating that the in-teractions between the IL and PMMA are weak and may led to the ILleaching from the gel matrix. Recently, Hashimoto et al. (2016) havedisclosed the preparation of tetra-armed polyethylene glycol based iongels with high-toughness using imidazolium based aprotic ILs ([C2mim][N(Tf2)]) with buffering properties (Hashimoto et al., 2016). The au-thors showed that by controlling the pH, the properties of the ion gelcan be manipulated. An ideal and homogenous polymer network wasformed when pH was controlled by adding aprotic ILs with bufferingcharacteristics, whereas the ion gel system without buffering ILsshowed discrete polymer network with irregular cross-linking(Hashimoto et al., 2016).

In order to fulfil the requirements of biomedical applications, effortshave been made on the development of new stimuli-responsive mate-rials using biodegradable and biocompatible molecules. Chitosan isconsidered as one of the most widely explored polysaccharide for awide number of applications (Roy, Cambre, & Sumerlin, 2010; Stuartet al., 2010). In order to prepare a suitable pH and electric responsivematerial for drug delivery applications, chitosan films loaded with ILsor salts, such as choline chloride and choline dihydrogenphosphate, anddexamethasone sodium phosphate, an anticancer drug, were prepared,followed by studies on the drug release kinetics at different pH values(Dias et al., 2013). The drug release kinetics showed that the release ofdexamethasone sodium phosphate does not occur by simple diffusion orswelling; it is instead controlled by the interactions occurring betweenthe chitosan film, dexamethasone sodium phosphate and choline di-hydrogenphosphate (Fig. 3).

In a similar line of research, DESs were explored to synthesize self-polymerizable ion gels for pH responsive drug release. In this work, self-polymerization of 2-hydroxyethylmethacrylate (HEMA) in a DES, namelyChCl-Fruc 2:1 (Fig. 1B), in the presence of indomethacin (anti-in-flammatory drug) led to the formation of a drug-immobilized nontoxicion gel with hemocompatible features. The ion gel thus obtained showeda pH responsive release of indomethacin. Moreover, the drug im-mobilized in the gel matrices can be stored at room temperature for along period (up to 6 months) without degradation (Mukesh, Upadhyayet al., 2016; Mukesh, Gupta et al., 2016). Self polymerization of HEMA inChCl-Or 1:1.5 (Fig. 1B) resulting in the formation of highly stretchableion gel with good capacitance behaviour was obtained (Mukesh,Upadhyay et al., 2016; Mukesh, Gupta et al., 2016). The use of DESs forthe preparation of pH responsive DNA based ion gels was also exploredby Mukesh& Prasad (2015). The formation of three different morphol-ogies of DNA was observed in a DES composed of ChCl-EG 1:2, which isknown to solubilize high concentrations of DNA (Mondal et al., 2013). In

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this work, DNA was converted into pH reversible ion gels, comprisingaggregated DNA structures (pH 2.98), spheroid shaped DNA micro-structured assemblies, and pH responsive DNA micro-hydrogels (pH7.30). The chemical and structural stability of DNA in all the formats

were confirmed. The micro-sized DNA assemblies as well the micro-hy-drogels are useful for applications in targeted gene or drug delivery.

Besides the use of co-polymers for the preparation of pH responsiveion gels, recently, pH responsive polymerized bio-ionic liquid based

Fig. 1. Chemical structure, abbreviation and references reporting onionic liquids (A) and deep eutectic solvents (B) used to prepare iongels.

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nanogel matrices were proposed for the delivery of an anticancer drug(Mukesh et al., 2014). pH-responsive nanogel systems with an averagehydrodynamic size of 41 ± 15 nm were developed by the simulta-neous polymerization and cross-linking of a polymerizable bio-basedionic liquid [Ch][acrylate] (Fig. 1A). The nanogel thus obtained showed

prolonged drug delivery (10 days) for an anticancer drug (5-fluorour-acil) at pH 1.2 (stomach pH) at the physiological human body tem-perature (37 °C). Remarkably, no substantial drug delivery was ob-served at pH values of 5 and 7.4. The prolonged drug release profilemakes the reported nanogel system a potential candidate as drug

Fig. 2. Chemical structure of monomer units of polysaccharides and other polymers used for the preparation of stimuli responsive ion gels (a) cellulose (b) chitin (c) chitosan (d) xanthangum (e) agarose (f) guar gum (g) tamarind gum (h) poly(N-isopropylacrylamide) (i) poly(ethylene glycol)diacrylate (j) poly(propylene carbonate) (k) (Hydroxyethyl)methacrylate, (l)poly(methylmethacrylate) (m) P4VP–PEA–P4VP triblock copolymer, and (n) N,N-bisacrylamide.

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nanocarrier vehicles for the in vivo delivery of stomach specific ther-apeutic agents.

3. Thermo-responsive ion gels

Ion gels in which their properties can be tuned by temperature asexternal stimuli are known as thermo-responsive ion gels. Such ion gelshave several potential applications, such as in reversible water uptake,actuators, optical sensors, and in flexible optical devices (Noro,Matsushima, He, Hayashi, &Matsushita, 2013; Noro et al., 2008;Benito-Lopez, Antonana-Diez, Curto, Diamond, & Castro-Lopez, 2014;Ru, Wang, Zhang, Yu, & Li, 2013; Mine, Prasad, Izawa,Sonoda, & Kadokawa, 2010).

Temperature responsive viscoelastic properties of supramacromole-cular ion gels were prepared in an IL, [C2mim][N(Tf2)] (Fig. 1A), via H-bonding between the end blocks of a triblock copolymer, namely poly (2-vinylpyridine)-b-poly(ethyl acrylate)-b-poly(2-vinylpyridine) (P2VP-PEA-P2VP) (Fig. 2), and a homopolymer (Noro et al., 2008). The pre-pared ion gel showed reversible gel like properties up to 141.5 °C; abovethis temperature it behaves like a viscous liquid. The authors have alsoshowed that the relaxation time of the system increases by∼11 times oncooling from the gel point (141.5 °C) down to 30 °C. The IL was found tomaintain good solvent conditions over a wide range of temperatures. Ruet al. (2013) have explored IL-modified alkoxysilane based thermallyreversible, flexible, and luminescent ion gels in presence of lanthanideions (Eu3+and Tb3+). An amino-functionalized ionic liquid ([NH2-C4im]Br) (Fig. 1A) was used for the study. The thermal reversibility of the iongels was tested by a heating-cooling process, whereas the materialshowed fluid nature at 60 °C, and upon cooling down to room tem-perature the ion gel property was restored and the process was shown tobe repeatable for several times. Such thermo-responsive luminescentmaterials have potential application towards the development of flexibleoptical devices. Thermo-reversible supramolecular polymer ion gels werealso prepared by Noro et al. (2013). The reported ion gels were preparedby mixing a poly (4-vinylpyridine)-b-poly(ethyl acrylate)-b-poly(4-vi-nylpyridine) (P4VP−PEA−P4VP), a triblock copolymer (Fig. 2) and zincchloride in presence of the IL [C2mim][N(Tf2)] (Fig. 1A). The authorshave showed that the metal-ligand coordination between zinc and pyr-idine group in the presence of IL was the driving force towards the for-mation of gel as confirmed by FT-IR spectroscopy. Heating-cooling ex-periments showed that the material exhibited thermo-reversibleviscoelastic properties between a gel-like state and a liquid-like state.

Benito-Lopez et al. (2014) explored a thermo-responsive ion gelwith improved actuation behaviour in comparison to its hydrogelcounterpart. This ion gel was prepared by encapsulating [C2mim]

[EtSO4] into a cross-linked poly(N-isopropylacrylamide) matrix. Theauthors showed that the ion gel has temperature responsive reversibleswelling and shrinking behaviours, whereas the corresponding hy-drogel did not show any of such behaviours.

Apart from the use of polymers and copolymers of synthetic originfor designing thermo-reversible ion gels, ion gels based on biopolymerswere also reported. Guar gum (GG) is a galactomannan extracted fromthe seeds of the leguminous shrub Cyamopsis tetragonoloba, and che-mically made up of (1,4)-linked β-D-mannopyranose main chains with abranched α-D-galactopyranose unit at C-6 (Fig. 2). [C4mim]Cl (Fig. 1A)was used for the gelation of GG (15% w/w) by a heating-cooling pro-cess followed by protic solvent treatment to obtain hard gel materials.The obtained hard material was compressed (ca. 10 MPa) to get stablethin films. These films showed a fracture stress of 17 MPa with a frac-ture strain of 18.2% under tensile mode and became hard on heating,meaning that they could be arranged to a target shape at higher tem-peratures (Fig. 4). This high temperature shapeability was essentiallydue to the switching of crystalline GG at higher temperatures toamorphous GG at room temperature (Prasad, Izawa et al., 2009; Mineet al., 2010). The same ionic liquid, i.e. [C4mim]Cl, was used to preparefunctional ion gels of xanthan gum (Izawa and Kadokawa, 2010). Theion gels thus obtained showed good mechanical properties and ther-mally induced shape-memory effect. The specific association of the IL inthe gels and interaction of the imidazolium cation with xanthan gumchains were suggested as the main reasons behind the production ofsuch unique functional gels. Another IL, namely [C4mim]CH3COO(Fig. 1), was used as solvent media to produce acetylated and carba-nilated agarose, which was found to be thermoreversible. Furthermore,the researchers were able to prepare self-healing ion gels of agaroseusing mixture of ILs (Trivedi, Bhattacharjya, Yu, & Kumar, 2015).

4. Solvent-responsive ion gels

Guar gum (GG) ion gels and their nanocomposite gels incorporatingmultiwalled carbon nanotubes (MWCNT) with self and solvent re-sponsive healing abilities were explored by our group (Sharma et al.,2013a, 2013b). In this work, ion gels in [C4mim]Cl were prepared inthree different concentrations of GG (3, 5 and 10% w/v) by a heating-cooling process. The gels were bisected and kept one upon another oraligned horizontally with each other in close vicinity at room tem-perature (30 °C). It was observed that the ion gel with 10% w/v of GGwas self-healed after 6 h of standing (Fig. 5A). Further, a composite gelwas prepared in presence of 0.2% MWCNT with respect to [C4mim]Cl,and the self-healing of the gel was observed after 5 h of standing atroom temperature. The unified gel structure was bisected and kept one

Fig. 3. Prevalence of ionic charges on chitosan and dexamethasone sodium phosphate according to the pH.

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upon another. The gel pieces self-healed and the process was repeatedfor 3 consecutive cycles. No more cycles were achieved, may be due tothe loss of the IL mobility. [C4mim]Cl was used with a dual function-ality since it behaves as a solvent and as a junction promoter that tiesguar gum chains leading to the formation of network structures similarto physical gels. These cross-linking structures are not regular but ra-ther form extended junction zones due to cooperative effects. The threedimensional network upon application of mechanical strain (highershear rate) was disturbed, but upon release of the force, reconstitutionof the three dimensional structure took place responsible for the self-healing of the gels. The area of the hysteresis loop formed for the GG-[C4mim]Cl gel was 2.33 × 105 Pa/s, which is lower than that of GG-[C4mim]Cl/MWCNT (7.9 × 105 Pa/s), explaining the quicker response

of the later to mechanical strain towards self-healing. The area of thehysteresis loop after three consecutive cycles was almost the same, in-dicating the ability of the materials to self-heal after bisections for threetimes.

The prepared GG ion gels in [C4mim]Cl were bisected and alignedhorizontally with each other in close vicinity at room temperature,followed by placing the gel pieces in polar aprotic solvents, namelyacetone, DMSO, DMF and acetonitrile. The ion gel with 10% w/v of GGhealed after 1.5 h of exposure to acetone, DMSO and DMF (Fig. 5B);however, no healing was observed in acetonitrile. It should be notedthat the ion gels of other polysaccharides, such as agarose, κ-carra-geenan and xanthan gum did not show healing behaviour in the thesepolar aprotic solvents. It was proposed that the branched

Fig. 4. Temperature induced shapeability in guar gum ion gel film.

Fig. 5. Self and solvent responsive healing ability forguar gum ion gel.Reproduced with permission from the Royal Societyof Chemistry.

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galactopyranose unit of GG may play an important role in the healingprocess. All the other polysaccharides mentioned above are straightchain polysaccharides, with the exception of xanthan gum which con-sist of bulky side chain. The bulky side chain possibly did not favour thephysical cross-linking required for gelation (xanthan gum side chain:C24H36O18; GG side chain: C6H12O6). On the other hand, the nano-composite ion gel prepared with MWCNT healed upon 1 h of exposureto the same aprotic solvents.

5. Shear-responsive ion gels

Shear responsive ion gels based on Tamarind gum and IL were re-ported by us (Sharma, Mondal, Mukesh, & Prasad, 2014). Tamarindgum (TG) is a natural polysaccharide, extracted from the endosperm ofthe seeds of Tamarindus indica Linn (Glicksman, 1986) (Fig. 2). Che-mically TG is composed of β-(1,4)-D-glucan backbone substituted withside chains of α-(1,4)-D-xylopyranose and (1,6) linked [β-D-galacto-pyranosyl-(1,2)-α-D-xylopyranosyl] to glucose residues, where glucose,xylose, and galactose units are present in the ratio of 2.8: 2.25: 1.0 asthe monomer units (Gidley et al., 1991). TG is widely used as a thick-ening, stabilizing, emulsifying and gelling agent in the food and phar-maceutical industries (Zhang, Zeng, Zhang, Wang, &Wang, 2006).Stable TG ion gel was obtained for 10% w/w TG in [C4mim]Cl (Sharmaet al., 2013a, 2013b).

In order to investigate the recovery of storage modulus (Ǵ) of theion gels upon relaxation, the gels were fractured employing high strainfollowed by relaxation. The ion gel prepared in [C4mim]Cl was initiallysubjected to 1% strain at 1 Hz frequency for 300 s and Ǵ and Ǵ́ weremonitored during the process and by fracturing the gels employing ahigh strain at 1 Hz for 100 s. The storage modulus for TG-[C4mim]Cl gelrecovered the original value upon relaxation. The cycle could be re-peated for 10 consecutive times. On the other hand, the hydrogel didnot show structure recovery upon relaxation indicating absence ofthixotropic nature.

6. Magnet-responsive ion gels

Magnetically responsive ion gels have potential applications due totheir inherent magnetic behaviour (Yuan, Venkatasubramanian,Hein, &Misra, 2008). In general, magnetic ion gel materials are pre-pared by the incorporation of magnetic substances onto to polymermatrix or use of magnetic ILs (Ziółkowski et al., 2012; Xie et al., 2010).Ziółkowski et al. (2012) reported the synthesis of magnetic ion gelsusing organosilane-coated iron oxide nanoparticles, N-iso-propylacrylamide and a phosphonium-based IL. The authors showedthat after modification of magnetic nanoparticles with silane, the iongel became homogenous and also prevents the leaching of nanoparticlesfrom the gel matrix. Furthermore, the IL imparted optimal mechanicalstrength and healing properties to the gel. Such magnetic ion gels withimproved mechanical stability and flexibility respond differently toexternal permanent magnets and therefore have potential to be em-ployed as soft magnetic actuators.

The same research group have also studied the suitability of an iron-containing magnetic IL, [C4mim][FeCl4], towards the formation oftransparent, ion-conducting and paramagnetic ion gels by mixing itwith suitable amounts of polymethyl methacrylate in acetone (Xie

et al., 2010). The authors have studied the structural changes of the ILby UV–vis spectroscopy upon incorporation into the polymer matrixand showed that the coordination around the Fe (III) ion slightlychanged upon incorporation; nevertheless, the magnetic propertieswere similar to the pure IL. Such magnetic ion gels are exciting pro-totypes for flexible and mechanically stable materials for various ap-plications.

In addition to the stimuli responsive ion gels discussed above, otherfunctional ion gels based on polysaccharides were prepared by variousresearchers. Cellulose and [C4mim]Cl are amongst the most studiedpolysaccharides and ILs respectively, to prepare ion gels. It was shownthat high concentrations of the polysaccharide (15% w/w) in the IL canlead to the formation of a transparent ion gel upon standing (Kadokawa,Murakami, & Kaneko, 2008; Takada& Kadokawa, 2015) (Fig. 6).

The ion gels prepared using guar gum and [C4mim]Cl or anypolysaccharide based ion gels prepared using ILs have an inherentproblem – the leaching of the IL from the polymer matrices. Zhang et al.(2017) have recently overcame this situation by preparing a self-standing stable guar gum film using [C4mim]Cl and an imidazoliumbased polymerisable ionic liquid (PIL), namely poly-(1-[2-acryloy-lethyl]-3-methylimidazolium bromide. The combination of GG and thePIL provided an excellent dimensional stability to the ion gels with noexpulsion of the IL, even after long term standing. Such ions gels couldalso display high thermal stability and electric conductivity. PILs werealso used to prepare chitin/cellulose thin films by solubilising a mixtureof polysaccharides in [C4mim]Cl, followed by the addition of PILmonomers. The free radical polymerization of the later resulted in theformation of thermally stable thin films (Setoyama, Kato,Yamamoto, & Kadokawa, 2013).

7. Conclusions and future prospects

In this review, stimuli responsive ion gels prepared using polymers,co-polymers, biopolymers and polysaccharides in ILs or DESs areoverviewed and discussed, particularly ion gels responsive to externalstimuli, such as pH, temperature, stress, magnetism and solvents, andself-healing gels. In most of the reported studies, ILs have been used asgelling media to provide high ionic conductivity and high temperaturestability and flexibility. On the other hand, in spite of having promisingfeatures, less attention has been given to DESs for designing suchfunctional ion gels. DESs are considered as more benign alternativesover ILs due to the possibility of their formation using bio-basedstarting materials. Due to the remarkable properties of ILs or DESs toprepare stimuli responsive ion gels, other and wider stimuli responsiveproperties, for instance multi-stimulus and bio-stimulus responsive,deserve to be investigated in the near future. The unique characteristicsof the reported stimuli-responsive ions gels based on ILs or DESs makeof them remarkable materials for applications in electrochemistry, insensing devices and as drug delivery vehicles, for which deeper in-vestigations on their potential applications and their scale-up still standout as issues to accomplish.

Acknowledgements

KP thanks council of scientific and industrial research (CSIR), NewDelhi for the CSIR-Young scientist Awardee project (CSIR-YSP/2011-

Fig. 6. Preparation of cellulose ion gel in [C4mim]Cl.Reproduced with permission from authors.

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12) and overall financial support for the research work. MG Freire ac-knowledges the funds received from the European Research Councilunder the European Union’s Seventh Frame work Programme (FP7/2007-2013)/ERC grant agreement no. 337753. CSIR-CSMCRI commu-nication No. 033/17. JB acknowledges CSIR for a senior research fel-lowship.

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